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Hang Liu, Sanjairaj Vijayavenkataraman, Dandan Wang, et al.

[1]
large diameter fibres ranging from 180–1000 μm . sions that grow into spiral loops with a thin fibre di-
Electrohydro-dynamic (EHD) spinning technology ameter.
involves high speed non-linear electrohydrodynamics, To achieve aligned electrospun fibres, various types
complex rheology, and transport of charge, mass, and of electro-spinning setup with different dynamic col-
[9]
heat within the jet. The process consists of three stages: lectors have been developed , such as disc collec-
jet initiation, jet elongation with or without branching tor [10] , rotating drum with wire [11] , and rotating tube
[12]
and/or splitting, followed by solidification of jet into with knife edge electrodes . One of the first in-
nanofibers. High resolution fibres can be produced by stances of aligned electro-spun fibres was demon-
this technique to mimic the nano-topographical ele- strated by Theron et al.; a thin rotating disc was used
ments in the extracellular matrix (ECM) [2-4] . The re- to collect the fibres [10] . Biodegradable nanofibrous
sultant micro-metre/nanometre fibres are usually dis- scaffolds with aligned poly (l-lactic-co-e-caprolactone)
[5]
ordered and pore size less than 20 μm . The fabri- [P(LLA-CL)] copoly-mer have been produced using
this electrospinning setup for blood vessel engineering
cated electrospun meshes are in a non-woven form, application. Baker et al. applied an electrospinning
applied in wound dressing [6,7] . Continuous single fibre setup with a rotating drum/mandrel to collect aligned
or uniaxial fibre bundles are required in tendon, mus- electrospun fibres for scaffold fabrication . An elec-
[13]
cle, cartilage, and meniscus replacement, in which the
collagen fibres are organized either parallel or perpen- trospun mat, with the majority of fibres in one direc-
tion, was seeded with cells; the cells were observed to
[8]
dicular to the surface of tissues . Scaffolds with attach and grow along the prevailing fibre direction in
highly aligned fibres usually possess a greater level of the in vitro study.
[3]
mechanical anisotropy , which is preferred in soft Near-field electrospinning process (NFES) , is
[14]
tissue engineering. Having accurate fibre control on proposed to orientate micro and nano fibres via stage
physical properties and patterns is critical for fabrica- control where the electrode is positioned close to the
tion of biomimetic structures. substrate. Several studies have focused on NFES to
The traveling liquid jet stream is subjected to a va- evaluate substrate effects [15] , patterns [16] , and parame-
riety of forces with opposing effects and as a result, ters optimization [17] . However, these achievements are
various fluid instabilities also occur. The jet may un- only limited to 2D graphic and patterning applications.
dergo splitting into multiple sub-jets in a process A quick solidification of fibres over very short dis-
known as splaying or branching [28] . This happens tances between the nozzle and collector is normally
when changes occur in the shape and charge per unit required to build 3D structures. An EHD hot jet plot-
area of the jet due to its elongation, and evaporation of ting technique has been applied to fabricate high res-
the solvent. The splaying or branching process shifts olution (i.e., a fibre diameter below 10 μm) 3D scaf-
the balance between the surface tension and the elec- folds [18] . This method is not applicable to tempera-
trical forces, and the jet becomes unstable. In order to ture-sensitive materials, such as collagen and growth
reduce its local charge per unit surface area, the unst- factors, or high melting point materials.
able jet ejects a s maller jet from the surface of the Park et al. introduced computer control into a self-
primary jet. However, the key role in reducing the jet developed EHD printing system to print complex pat-
diameter from micrometre to nanometre is played by terns using various inks that are in industrial use,
whipping instability, which causes bending or stret- ranging from insulating and conducting polymers, to
ching of the jet. When the polymer jet becomes very solution suspensions of silicon nanoparticles and rods,
long and thin, the time required for excess charge to to single-walled carbon nanotubes [19] . Kim et al. expe-
redistribute itself along the full length of the jet be- rimented materials and operating conditions for
comes longer. The location of the excess charge then high-resolution printing of layers of quantum dots
tends to change with the elongation. The repulsive with precise control [20] . Lee et al. investigated the
coulombic forces between the charges carried with the generation of highly aligned and patterned silver na-
jet elongate the jet in the direction of its axis until the nowires (Ag NWs) using EHD-jetting [21] .
jet solidifies. This leads to an incredibly high velocity In this work, an EHD-jetting system was developed
at the thin leading end of the straight jet. As a result, to explore the relationship between processing para-
the jet bends and develops a series of lateral excur- meters and fibre characteristics. High resolution scaf-

International Journal of Bioprinting (2017)–Volume 3, Issue 1 73